1. Field of the Invention
This invention relates to catalysts for the oxidation and removal of heavy metals from effluent gases, and more specifically, this invention relates to metal catalysts and partially combusted coal as catalysts for the removal of heavy metals such as mercury (Hg) from high temperature gases generated from sources such as coal gasifiers, coal-fired electrical generating plants, ore smelters, oil- or petroleum resid-fired combustors, and various waste incinerators.
2. Background of the Invention
Certain effluent gas streams contain many toxic pollutants, including the heavy metals mercury and cadmium. Coal-burning power plants, incinerators, oil-burning boilers and power plants, refuse-derived fuel power plants, and gasification systems (especially integrated gasification combined cycle power generation systems) are sources of effluent streams with mercury and other heavy metals. The combustion of low-rank coals such as Powder River Basin sub-bituminous coal and lignites have been shown to form flue gases where the mercury is primarily in the elemental form.
Mercuric or mercury (II) chloride, HgCl2, is highly soluble in water, and is more readily removed by carbon sorbents. Therefore, agents that can oxidize elemental mercury to mercury (II) chloride (or another oxidized mercury compound) would be of considerable value in effluent clean-up. Such agents would enable mercury to be captured by existing air pollution control devices (APCDs) present at coal-burning power plants. Typical APCDs include wet scrubbers for acid gas removal (SOx and NOx), as well as electrostatic precipitators (ESPs) and baghouse filters for particulate removal.
Ideally, the mercury oxidation catalysts would be located upstream of the appropriate APCD. Mercury (II) chloride is readily removed by the scrubbing solutions employed for acid gas removal and/or by adsorption on unburned carbon in fly ash captured by ESPs or baghouse filters. Mercury (II) chloride is also sequestered by activated carbon sorbents injected upstream of an ESP or baghouse.
Much of the mercury contained in power plant flue gas is in the elemental form. Elemental mercury is a semi-noble metal, insoluble in water, and is not efficiently captured by carbon. As such, elemental mercury is difficult to capture; oxidized mercury is more amenable to capture.
Many technologies are being developed for the control of mercury emissions from flue gases. These methods employ sorbents, catalysts, scrubbing liquors, flue gas or coal additives, combustion modifications, barrier discharges, and ultraviolet (UV) radiation. Efforts have been made to develop catalysts for selectively oxidizing elemental mercury in effluent streams. However, selective catalytic reduction (SCR) catalysts, whose primary role in flue gas is the reduction of nitrogen oxides, achieve only 50 percent oxidation levels for mercury oxidation. SCR catalysts have not yet been optimized for mercury oxidation and removal.
Due to its relative simplicity and proven successful application for the incinerator market, most of the research has focused upon the use of activated carbon sorbents for the adsorption of mercury.
Gold (Au) and palladium (Pd) catalysts are used as mercury oxidation catalysts. However, both Au and Pd catalysts are not inert and therefore degrade in flue gases.
Thief carbons and their adsorption capabilities are described in U.S. Pat. No. 6,521,021 awarded to Pennline et al., on Feb. 18, 2003.
U.S. Pat. No. 6,136,281 awarded to Meischen, et al. on Oct. 24, 2000 discloses a gold catalyst for the oxidation of elemental Hg in effluent gas streams. The presence of Cl2 and HCl in the effluent gas streams aids in the oxidation of elemental Hg.
None of the aforementioned patents discloses catalysts with both high mercury adsorption levels, high adsorption levels for HCl and Cl2, and with superior anti-corrosion characteristics. In addition, none of the aforementioned patents disclose catalysts which are self-activating and disposable.
A need exists in the art for catalysts with both high mercury adsorption levels and high adsorption levels for HCl and Cl2. The catalysts should exhibit superior anti-corrosion characteristics. Furthermore, the catalysts should be self-activating and disposable.